(1) Field
The invention relates to a radio tag capable of indicating to a reader, via a wireless link, that a variation in energy has crossed a predetermined threshold, this variation in energy being chosen from the group consisting of a variation in the temperature of the radio tag and a variation in the magnetic field in which the radio tag is immersed. The invention also relates to an assembly comprising this radio tag and a reader.
(2) Description of the Related Art
Radio tags are also known as RFID (Radio Frequency Identification) tags.
Known radio tags comprise:                an electrically insulating substrate,        an antenna to set up the wireless link with the reader, this antenna being entirely deposited and affixed without any degree of freedom onto the insulating substrate, and        an actuator capable of modifying the impedance of this antenna when the energy variation crosses the predetermined threshold in order to indicate this event to the reader, this actuator comprising a transducer material for this purpose capable of transforming the energy variation into a mechanical deformation, the transducer material being chosen from a group made up of a thermal shape-memory material, a magnetostrictive material and a magnetic shape-memory material.        
For example, a radio tag of this type is disclosed in the following article: R. Bhattacharyya et al: “RFID Tag Antenna Based Temperature Sensing in the Frequency Domain”, RFID, 2011, IEEE Conference on RFID. This article is referred to as article A1 in this description below.
Prior art is also known from: US2004/061655A1. US2008/007253 and US2010/079288A1.
In article A1, the actuator comprises a plate made from electrically conductive material affixed to an arm made from a thermal shape-memory material. In the description below, unless otherwise indicated, the term “shape-memory material” refers to a thermal shape-memory material, i.e. a material which is suddenly deformed when its temperature crosses a temperature threshold ST. This temperature threshold is also referred to as the “transition temperature” of the material. For simplification, hysteresis phenomena are ignored here in such a way that the transition temperature is assumed to be the same when the shape-memory material is heated and when it is cooled.
When the temperature is less than the threshold ST, the arm holds the plate in a position distanced from the antenna. In the distanced position, the plate is around 1 cm below the antenna. If the temperature of the radio tag exceeds the threshold ST, the arm becomes deformed and moves the plate to a close position. In the close position, the plate is around 3 mm below the antenna. The presence or not of the plate close to the antenna modifies its impedance. The modification of the impedance of the antenna is detectable by the reader. The reader can thus know whether the temperature of the radio tag has or has not exceeded the threshold ST.
Radio tags of this type that are capable of indicating whether a temperature threshold has been exceeded have many applications. For example, they can be used when they are affixed to a frozen product to check that the cold chain has been respected and that the temperature of the frozen product and therefore that of the radio tag has never exceeded the threshold ST. Obviously, the field of application of radio tags of this type is not limited to frozen products. They can also be used on any object whose temperature must be monitored at one time or another.
In article A1, the antenna is simple to manufacture since it is entirely deposited onto the insulating substrate. Many manufacturing methods are then usable in order to manufacture it simply. For example, the antenna can be manufactured by depositing a conductive layer onto the substrate then by etching this conductive layer. The antenna can then be implemented through localised jetting of a conductive ink onto the substrate. In fact, in order to manufacture the actuator, a mobile plate and a mobile arm must be assembled in the same housing. Furthermore the vertical size of the actuator is relatively large due to the vertical movement of the plate.
A different embodiment of a radio tag of this type has been proposed in the following article: S. Caizzone et al: “Multi-chip RFID Antenna Integrating Shape-memory Alloys for Detection of Thermal Thresholds”, IEEE 2011. However, this radio tag is also complex to implement since the manufacture of an electrical switch from shape-memory material is no simple matter, as the author of this article himself underlines.
A different possible embodiment is also disclosed in the application JP2009162700 from HITACHI®. In this patent application, the antenna comprises a part made from shape-memory material. This part of the antenna is unfolded and alternately folded up on itself. For this purpose, it is anchored, on one side, in a wall and is mounted in a cantilevered manner inside a cavity. Thus, in the application JP2009162700, the antenna is not entirely deposited onto an insulating substrate in such a way that the simple manufacturing methods normally used cannot be employed. Thus, as previously, this method of manufacturing the radio tag is complex to implement.
The prior art described above relates to radio tags capable of detecting the crossing of a temperature threshold ST. However, the same production problems are encountered with a radio tag capable of detecting a variation in the magnetic field in which it is immersed.